Note that B67 says "you may roll against HT + Magic Resistance to resist the effects of magical elixirs" but DF1 says " Targets with Magic Resistance must fail a HT + MR roll for drinkables or fumes to affect them." The specific genre rule - the one in DF - absolutely trumps the general case - the one in Basic - in my game. Must fail trumps may roll. If you want to guzzle healing potions while under Magic Resistance, you need the Improved version.